Like he said, Adobe Photoshop has teh save for web option that lets you set a target filesize, and it works very good. I also like JASC Paintsho Pro, that also has a JPEG optimizer that will let you save to a certain filesize, while maintaining the best possible quality at that filesize. I usually start at a resolution of 700 to 800 pixels wide, then let one of the above mentioned programs do the rest to get it to around 98 or 99 kilobytes, so I can upload it here. There are a few freeware editors that will get you to the same end result, it just might be a longer process, but the price is right.
I can't think of the names of the few I liked before, but I'll look for them and post them when I find them.